RS-3 units #4115, #4106, #4107, #4113, #4122, #4112, #4119, and #4128 were rebuilt by Morrison-Knudsen into RS-3m units (numbered #501-508, respectively) between December of 1975 and March of 1976. The locomotives were re-engined with the 2,000 horsepower ALCo 12-cylinder 251C engine, and the long hood was raised six inches to accommodate the larger engine. The dynamic brakes were also moved to a box behind the cab.

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